In spectrophotometry, radiation from a source passes through a sample cell to a photodetector which measures the amount of radiation absorbed by the sample fluid in the cell. The output of the detector is a measure of absorbance at a particular wavelength of radiation. The quantitative presence of certain materials in the sample is identified by particular wavelengths characteristically absorbed by the materials. An important use of spectrophotometric detectors is in chromatography wherein the components of a sample are separated in a column and the radiation absorbance of the separated components are then measured by a spectrophotometric detector.
In such detectors, radiation transparent optical windows allow radiation from the source to pass through the cell to the detector. In a common spectrophotometric detector, radiation passes through an entrance window, through the cell in a direction parallel to the flow of sample fluid through the cell, and through an exit window to the detector. Flat windows or plano convex lenses typically have been used. U.S. Pat. No. 4,192,614 to deMey et al. shows a detector assembly with flat windows at the entrance and exit openings in the cell. A lens focuses the radiation in a pattern which converges in the cell.
Another type of commonly used detector has divergent optics with sample fluid flow across a substantially planar radiation field in the cell. Such crossflow cells are typified by the Milton Roy LDC microcell used in conjunction with the LDC Model 1204D spectoMonitor detector. Crossflow cells permit close coupling of the cell to the outlet end of a chromatographic column.
Another type of crossflow cell is available from Guided Wave Inc. of California. The Guided Wave cell comprises a standard one quarter inch cross union adapted to receive opposing fiber optic transmission probes. The probes contain an external sapphire window sealed into a one quarter inch metal tube. The metal tube also contains a suitable collimating lens. The tube which can be made from various materials including 316 stainless steel, Monel 400 or Hastelloy C276 alloys is sealed to the sapphire window with a soft glass frit. The glass frit is fused to prevent leaks between the sapphire window and the inner wall of the one quarter inch tube. An additional epoxy seal is used to coat the outer surface of the fused soft glass seal.
While the Guided Wave flow cell can be used in many services, it cannot be used in a hydrogen fluoride atmosphere or in a hydrogen fluoride, hydrogen chloride, chlorine atmosphere or in the presence of a strong caustic or in any other service where the epoxy and glass seal would be attacked by the materials flowing through the cell.
This application is related to co-pending application Ser. No. 07/546,592 filed in the name of the same inventors. Application Ser. No. 07/546,592 discloses a flow cell for use in highly corrosive environments which is constructed from a cross union and contains opposing fiber optic probes each with an external sapphire window which is sealed into a metal tube contained in each probe with melted glass and an optional plastic seal over the melted glass. Each portion of the cross union containing a probe has a right angle shoulder which abuts the end of the tube containing the sapphire window An o-ring gasket is positioned between the right angle shoulder and metal tube to form a tight cover over the glass seal and plastic seal when the probes are assembled in the flow cell, thereby protecting the glass seal and plastic seal from the corrosive environment. It is stated in the application that probes may be installed in such a cross union at any distance up to two or four centimeters apart in a one quarter inch or one half inch cross union respectively.
It is desirable to provide a flow cell having an optical path length substantially greater than the path length of the cross union disolosed in Ser. No. 07/546,592 which can be used in such corrosive environments and in particular, in such environments at elevated temperatures.